Control of oscillatory currents



Patented Apr. 19, 1932 UNITED STATES JOSEF HOFMANN, OF MERION, PENNSYLVANIA CONTROL OF QSCILLATORY CURRENTS Application filed September 26, 1928, Serial No. 308,508, and in Great Britain September 26, 1927.

I 'The present invention relates more particularly to compensating arrangements for use in connection with amplifying circuits, such as are commonly employed in broadcasting, long distance telephone repeaters or the like.

Owing to the great degree of variability in the originating source of energy which may be speech, musical instruments, and sounds of all kinds, and owingalso in several cases to the varying distances of the receiving microphone fromv thesource of sound it is found that the variations of current when amplified are so great that it is necessary to compensate so as to prevent excessive currents being transmitted, currents which reach the limits of the receiving apparatus and produce most unpleasant sounds unrelated to the c source of origin and may in fact overload the amplifying apparatus and damage the instruments. p v I v To avoid this danger, it is usual to have observers continually onthe alert during the transmission, whose duty it is to compensate for excessive load so as to reduce the output to reasonable dimensions.

The object of the present invention is to provide means whereby such compensation can be eflected automatically in a much more reliable and satisfactory manner than is possible by manual means.

According to the invention this is efi'ected by connecting in the transmitting current either in parallel or in series with the current to be transmitted or repeated a device adapted'to respond variably according to the amplitude of the alternating or oscillating currents and arranged to control a regulator which automatically compensates for variations in the amplitude of said currents. It will be appreciated that the compensation must not be complete, or otherwise the received sounds would all be of the same strength, but it is readily possible by suitably choosing the degrees of regulation to permit of considerable variations of strength while effectively preventing these ever reaching an excessive value.

One construction for carrying the invention into effect will be given by way of example, although it will be understood that considerable modifications may be made in the constructional features without departing from the spirit of the invention.

The drawing illustrates in simple diagrammatic form a microphone or its equivalent 1, which responds to the source of sound and may be either a broadcasting instrument or an ordinary subscribers telephone, an amplifier 2, which may be of three or more stages, but which is shown provided in a case with an electrically driven hot air exhauster 3 the purpose of which is to keep the amplifiers within such a range of temperature that the 7 degree of amplification is maintained substantially constant, (other means could be provided but this arrangement is found to give sufiiciently satisfactory results) and a sending apparatus 5 such as is well known for super-imposing the audio-frequency currents onto the radio-frequency current transmitted to the antennas 4 for broadcasting. In the parallel circuit with the sending apparatus 5 is a solenoid 6. This solenoid consists of a fixed core 7 and a movable core 8.

The fixed core is tubular, so that a rod 9 extending from the movable core can pass therethrough to operate an arm 10 or its equivalent. a variable amount depending upon the extent to which the core is attracted. The core 7 is fixed to a magnet yoke 11 shaped as shown and forming a closed magnetic circuit at one side of the solenoid only, being open at the other as will be seen from the drawing. provided with a hole 12 to permit the movable core 8 to move freely up and down.

Surrounding the cores is a bobbin 13 of insulating material on which are two windings 1s and 15. The inner winding forms At its upper limb the yoke 11 is S the polarizing winding and is connected directly across the poles of a battery 16. The resistance and turns are suitably selected to give a strong polarizing field. The outer winding forms the exciting winding and is connected in the alternating current or oscillatory current circuit, the current in which has to be detected or measured.

The soft iron yoke 11 is secured to a support 17 carrying at its upper end an adjus ing screw 18 by which the tension of a helical spring 19 supporting the movable core'8 can be adjusted. The movable core 8 is provided with a collar 20 which engages a fixed ring 21 15 secured to the support 17. By suitably ad justing the tension of the spring 19 to balance the normal pull of the polarizing winding 14 the device is ready for operation.

The operation is as follows Speech, music, or other sounds are received on the microphone 1, which may be of any type usually employed in broadcasting or long distance telephony. The oscillatory or alternating current therefrom passes over the leads 22, 23, which are connected across the grid and filament of the amplifier 2. The magnified currents then pass over the leads 24, 25, to the sending apparatus 5. The lead. 25 does not, however, pass current directly to the sending apparatus, but passes via branch wire 26, fixed core 7, rod 9, aim 10, pivoted brush 27, variable resistance 28 to lead 29 and thence to the sending apparatus 5. The outer winding 15 of the solenoid is connected across the leads 24 and 25, and is therefore in a shunt circuit to the sending apparatus. By sending sufiicient direct polarizing current through the inner winding 14; of the solenoid and by arranging that the external circuit of the inner winding is of sufficientlv low impedance and further by suitably adjusting the tension of the light helical spring 19, the movable core 8 can be caused to move a variable amount depending upon the strength of the oscillatory current passingto the sending apparatus 5. At the end of the arm 10 brush 27 is pivoted which is pressed normally against the upper end of a resistance 28 by a spring 30. It will be clear that according to the extent that the movable core 8 is attracted downwards, more or less resistance will be inserted in circuit with the lead 25, and-consequently the greater the current, or rather, seeing that the outer winding of the solenoid is in a shunt circuit, the greater the voltage received from the amplifier the greater the amount of resistance which will be inserted, and consequently the output will be correspondingly reduced.

It is to be understood-that the insertion of resistance is only given as one example of how compensation may be efiected, but of course several alternative ways of modifying the output load of an amplifier are known,

and the invention would be equally applicable to any.

The instrument is particularly valuable for responding to high frequency oscillatory currents, that is to say, currents of audio-frequency and can be arranged to respond to amplified speech currents, to l\ .Iorse signals, or to the intensity of musical notes when converted into alternating currents by a microphone or like device. It should be understood that the polarizing winding 14.- should have its external circuit so adjusted usually of low resistance and impedance that it can react on the exciting winding, and permit the flux to increase with one half wave of exciting current and retard the fall with the other half wave so that there is a. steady increase to a value dependent on the strength of the current.

1. In apparatus for compensating for variations in strength of oscillatory currents, means to amplify the oscillatory current, an electro magnetic device responsive to said currents and having an exciting winding, polarizing means to render said device unidirectional, short circuited means to prevent a sudden increase of magnetization with one half wave of exciting current and to prevent its sudden decrease with the other half wave. 95

2. In apparatus for compensating for variations in strength of oscillatory currents,

a solenoid having an exciting winding supplied with such currents. a polarizing winding therefor, an external circuit containing afsource of current and connected to said polarizing winding, said external circuit having low impedance to alternating current, a core for said solenoid movable in the solenoid in response to variations in the oscillatory currents and a resistance included in the circuit of said currents by said core.

3. In apparatus as described, an exciting winding. a polarizing winding, an exciting circuit for the polarizing winding of low impedance to alternating current, said two windings being so located as to produce a common magnetic field, a core arranged in said field, a resistance controlled by said core, said resistance and exciting winding being in circuit with a source of oscillatory current of radio frequency.

4. Apparatus as claimed in claim 2 in which the core consists of a stationary por- 120 tion and a movable portion.

5. In apparatus as described, an exciting winding, a polarizing winding within the exciting winding, a direct current exciting circuit for the polarizing winding of I25 low impedance to alternating current, a core embraced by both windings and having a stationary portion and a movable portion,

a variable resistance controlled by said movable portion, said resistance and exciting winding arranged for connection in an oscillatory current circuit.

6. In apparatus as described, an exciting Winding, a polarizing winding therein, a soft iron yoke open at one side and embracing the windings, a local source of direct current for the polarizing Winding, a core having a stationary portion and a movable portion, and a variable resistance controlled by the movable portion, said exciting winding and resistance arranged for connection in an oscillatory current circuit.

In testimony that I claim the foregoing as my invention, I have signed my name hereto. J O SEF HOF MANN. 

